1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a magneto-optical recording method. More particularly, the invention relates to a magnetic-optical recording medium which are advantageously applicable to optical disks, optical tapes, optical cards and the like which can be optically recorded, reproduced and/or erased information thereon, and a magneto-optical recording method using thereof.
2. Description of the Prior Art
Magneto-optical recording techniques typically employ a recording medium having on its substrate a perpendicular magnetized film formed of a magnetic material, and achieve data recording and data reproduction on the recording medium in the following manner.
For the data recording, the recording medium is first initialized by application of a strong external magnetic field so as to be magnetized in one direction (upward or downward). Then, a record site on the recording medium is irradiated with a laser beam so as to be heated to a temperature near the Curie point or higher or near compensation point or higher of the magnetic material. Thus, the coercive force (Hc) in the site is brought to zero or substantially zero. Then an external magnetic field (bias magnetic field) having a magnetization direction opposite to the initial magnetization direction is applied to the recording medium to reverse the magnetization in the site. Upon stopping the irradiation with the laser beam, the recording medium is cooled to room temperature, so that the reversed magnetization is fixed. Thus, data is thermo-magnetically recorded in the record site.
For the data reproduction, the recording medium is irradiated with a linearly polarized laser beam. At this time, the plane of polarization of a reflected or transmitted light component is rotated in accordance with the direction of the magnetization (this phenomenon is known as "magnetic Kerr effect" or "magnetic Faraday effect"). The data readout is optically achieved by utilizing this phenomenon.
Much attention is now focused on the magneto-optical recording techniques for use in large capacity rewritable recording devices. As a reusable (or rewritable) recording medium, a so-called light modulation overwritable medium has been proposed which comprises an exchange-coupled duolayer film and achieves data overwriting with application of an initializing magnetic field (Hi) and a recording magnetic field (Hw) by modulating a light intensity. Another light modulation overwritable medium heretofore proposed comprises an exchange-coupled quadrilayer film and obviates the need for the application of an initializing magnetic field (Hi) (J. Magn. Soc. Jan., Vol. 17, No. S1 (1993) 357).
The light modulation overwritable medium employing an exchange-coupled quadrilayer film has a first magnetic layer 13, a second magnetic layer 14, a third magnetic layer 15 and a fourth magnetic layer 16 as shown in FIG. 19. The temperature dependence of the coercive forces of the respective magnetic layers is shown in FIG. 20. More specifically, the first, second, third and fourth magnetic layers have Curie points Tc1, Tc2, Tc3 and Tc4, respectively, which satisfy the condition of Tc3&lt;Tc1&lt;Tc2&lt;Tc4.
The transition of the magnetization states of the respective magnetic layers during the data recording operation. is explained by FIG. 21, wherein the arrows denote the magnetization direction of a transition metal contained in the magnetic layers.
At room temperature, the data recorded in the recording medium is represented by the magnetization state of the first magnetic layer 13, i.e., either an upward magnetization state (with a logic value of 0 as indicated at (a) in FIG. 21) or a downward magnetization state (with a logic value of 1 as indicated at (b) in FIG. 21). The magnetization of the fourth magnetic layer 16 is always oriented in one direction (upward as seen in FIG. 21), and the magnetization of the second magnetic layer 14 is oriented in the same direction as the magnetization of the fourth magnetic layer 16 with the aid of the third magnetic layer 15.
The data recording is achieved by irradiating the recording medium with a laser beam modulated into either a high power intensity or a low power intensity while applying a recording magnetic field Hw to the recording medium.
The high power laser beam is adapted to heat the recording medium irradiated therewith up to a temperature near the Curie point Tc2 of the second magnetic layer 14. The low power laser beam is adapted to heat the recording medium irradiated therewith up to a temperature near the Curie point Tc1 of the first magnetic layer 13.
Therefore, when the recording medium is irradiated with the high power laser beam, the second magnetic layer 14 as well as the first and the third magnetic layer 13,15 are demagnetized (as indicated at (h) in FIG. 21). The magnetization of the second magnetic layer 14 is reversed by cooling of the recording medium to a temperature below the Curie point Tc2 with applying the recording magnetic field Hw thereto (as indicated at (g) in FIG. 21), and then the reversed magnetization state is copied to the first magnetic layer 13 by interface exchange coupling in the course of cooling of the recording medium (as indicated at (e) in FIG. 21) to the temperature below the Curie point Tc1. Further, the magnetization of the third magnetic layer 15 and the second magnetic layer 14 is oriented in the same direction according to the magnetization of the fourth magnetic layer 16 in the cource of cooling to the room temperature (as indicated at (b) in FIG. 21). Thus, the magnetization of the first magnetic layer 13 is directed downward (with a logic value of 1).
When the recording medium is irradiated with the low power laser beam, the magnetization of the second magnetic layer 14 is not reversed (as indicated at (f) in FIG. 21) by the application of the recording magnetic field Hw because its coercive force is greater than the recording magnetic field Hw, and the magnetization of the first magnetic layer 13 is oriented in the same direction as the magnetization of the second magnetic layer 14 (as indicated at (c) in FIG. 21) by an interface exchange coupling force in the course of cooling of the recording medium. Thus, the magnetization of the first magnetic layer 13 is directed upward (with a logic value of 0). Indicated at (d) in FIG. 21 is a magnetization transition state from a state where the first magnetic layer 13 is downwardly magnetized (with a logic value of 0 as indicated at (b) in FIG. 21) to a state as indicated at (f) in FIG. 21.
A laser power employed for the data reproduction is significantly smaller than that of the low power laser beam and, therefore, does not change the recorded magnetization states of the respective layers.
Thus, the light modulation overwritable magneto-optical recording medium employing the exchange-coupled quadrilayer film obviates the need for the application of the initializing magnetic field Hi, and ensures stabilization of record bits. However, the magneto-optical recording medium is disadvantageous in that the application of the external recording magnetic field Hw is required for the light modulation overwriting.